Many new oxide materials have been developed for use as solid electrolytes and electrode materials in fuel cell and gas (e.g., oxygen) sensing technologies. In these devices, an anode and a cathode are positioned on either side of a solid electrolyte through which ions are conducted. Thus, the solid electrolyte must be ionically conducting and electronically insulating to prevent short-circuiting between the anode and cathode. Solid electrode materials must be both electronically conducting and ionically conducting. Ionic conductivity in oxide materials occurs when there is a defect such as an electronic vacancy in the crystal lattice of the material. The type of solid electrode and electrolyte materials depends on the type of electrochemical reaction taking place and the type of ion being conducted. Commonly, the device involves conductance of oxygen (O2−) ions. Typically, solid electrode and electrolyte materials used as oxygen ion conductors are metal oxides which have been doped or stabilized.
For example, oxides of lanthanide oxide and lanthanide perovskite materials are currently being explored as electrode or electrolyte layers for solid oxide fuel cells (SOFCs). Suitable anode materials include doped metal oxides such as transition metal- or lanthanide metal-doped cerias (e.g., copper-doped ceria, gadolinium-doped ceria, strontium-doped ceria, and yttria-doped ceria), metal-stabilized zirconia “cermets” (e.g., Ni-yttria-stabilized zirconia, Cu-yttria-stabilized zirconia, cobalt-stabilized zirconia, ruthenium-stabilized zirconia, etc.), and the like. The cathode materials typically are perovskite materials, for example, lanthanum strontium manganate, lanthanum strontium ferrite, lanthanum strontium cobaltite, yttrium manganate, calcium manganate, yttrium ferrite, and mixtures thereof. The electrolyte layer typically comprises stabilized zirconias (e.g., yttria-stabilized zirconia, partially stabilized zirconia), doped cerias (e.g., samaria-doped ceria, Ce0.8Sm0.2O1.9), stabilized bismuth sesquioxides, and the like. In some SOFC designs, the anode/electrolyte/cathode structure can further contain interfacial layers that act as buffers and/or interdiffusion barriers. Such materials can be mixed electronic/ionic conductors such as yttria-doped ceria or yttria-stabilized Bi2O3.
The manufacturing of gas sensor or SOFC devices presents several challenges. First, it is desirable that the solid electrolyte layer disposed between the anode and cathode layers be thin so as to reduce the operating temperature of the SOFC. In some cases, the SOFC material comprises multiple layers including a substrate layer, covered by a nitride layer, a thin film anode, a thin film electrolyte, and a thin film cathode. The fabrication of such layered materials is a major problem. In order to produce consistent thin films, it is necessary to provide a smooth and even surface onto which an electrode or electrolyte material can be deposited. WO 02/087002 A1 describes one method of manufacturing solid oxide fuel cells involving deposition of the anode and cathode materials into a well that has been etched into a dielectric or semiconductor substrate. In order to confine the cathode and anode materials to the area within the well and obtain ultra-thin electrolyte deposition layers, polishing has been suggested as a useful technique for removing excess cathode or anode materials and providing ultra-smooth surfaces.
Accordingly, there remains a need for a method of polishing substrates comprising lanthanide-containing metal oxide materials, such as those used in SOFCs and oxygen sensors, with high removal rate, low defectivity, and good selectivity. The invention provides such a polishing method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.